Sheet-like polyurethane moldings are used, in particular, for interior trim of vehicles. They can be produced by impregnation of porous, sheet-like materials with polyurethane systems and the subsequent reaction of these systems to give polyurethanes. Polyurethane systems which can be used are isocyanates, for example in the form of dispersions, or prepolymers containing isocyanate groups, the isocyanate groups then reacting with water. Atmospheric moisture and/or the moisture present in the porous, sheet-like materials may be sufficient for this reaction. The porous, sheet-like materials are customarily first introduced into the mold and there impregnated with the premixed polyurethane system.
Porous, sheet-like materials which can be used here are synthetic foams such as flexible or semirigid polyurethane foams, or fabrics such as glass wool or fabrics of natural fibers such as felts or plant fibers.
For molding and for accelerating the reaction, the sheets are usually placed in a heated mold after impregnation. During this process step the moldings can also ba provided with laminating materials such as films or mats. This gives moldings having a sandwich structure.
The known processes for producing sheet-like polyurethane moldings have disadvantages. When dissolved isocyanates are used, solvents are given off. To overcome this deficiency it is proposed in EP-A-276 465 that aqueous isocyanate suspensions be used for impregnating the porous materials. However, this process results in relatively long demolding times since evaporation of the water takes a long time.
When using polyurethane systems for producing sheet-like moldings, it is necessary for the systems to have a sufficiently long pot life in order to avoid the foaming prior to complete impregnation of the porous material. The viscosity of such polyurethane systems is usually very high. As a result, particularly when using sheet-like materials based on plant fibers, impregnation with the polyurethane systems can be incomplete. Flow problems occur when impregnating the materials in the mold.
It is an object of the present invention to develop a simple and inexpensive process for producing sheet-like polyurethane moldings which avoids the disadvantages of the prior art.